The present invention relates generally to intravascular devices. More particularly, the present invention relates to impedance-matching among segments and construction of a transmission line associated with such an intravascular device.
Tracking of catheters and other devices positioned within a body may be achieved by means of a magnetic resonance imaging (MRI) system. Typically, such a magnetic resonance imaging system may be comprised of magnet, a pulsed magnetic field gradient generator, a transmitter for electromagnetic waves in radio frequency (RF), a radio frequency receiver, and a controller. In a common implementation, an antenna is disposed either on the device to be tracked or on a guidewire or catheter (commonly referred to as an MR catheter) used to assist in the delivery of the device to its destination. In one known implementation, the antenna comprises an electrically conductive coil that is coupled to a pair of elongated electrical conductors that are electrically insulated from each other and that together comprise a transmission line adapted to transmit the detected signal to the RF receiver.
In one embodiment, the coil is arranged in a solenoid configuration. The patient is placed into or proximate the magnet and the device is inserted into the patient. The magnetic resonance imaging system generates electromagnetic waves in radio frequency and magnetic field gradient pulses that are transmitted into the patient and that induce a resonant response signal from selected nuclear spins within the patient. This response signal induces current in the coil of electrically conductive wire attached to the device. The coil thus detects the nuclear spins in the vicinity of the coil. The transmission line transmits the detected response signal to the radio frequency receiver, which processes it and then stores it with the controller. This is repeated in three orthogonal directions. The gradients cause the frequency of the detected signal to be directly proportional to the position of the radio-frequency coil along each applied gradient.
The position of the radio frequency coil inside the patient may therefore be calculated by processing the data using Fourier transformations so that a positional picture of the coil is achieved. In one implementation this positional picture is superposed with a magnetic resonance image of the region of interest. This picture of the region may be taken and stored at the same time as the positional picture or at any earlier time.
In a coil-type antenna such as that described above, it is desirable that the impedance of the antenna coil substantially match the impedance of the transmission line. In traditional impedance matching of MRI coils, shunt-series or series shunt capacitor combinations suffice to tune the coil. In such traditional applications, the capacitors almost never pose a size constraint. However, for intravascular coils, miniaturization of the tuning capacitors is necessary. Discrete components have been employed to construct matching and tuning circuits on intravascular devices. But such components are bulky and are not easily incorporated into the design of the device. Also, placement of the tuning capacitors away from the coil without a reduction in the signal-to-noise ratio (SNR) is desirable. It has been proposed to use open circuit stub transmission lines as a means of fabricating arbitrary or trimmable capacitors and to use short-circuited stubs as tuning inductors. Such probes are tuned by trimming the length of the coaxial cables. However, these circuits still result in a relatively large device that is not ideal for intravascular navigation. Also, the circuits require many connections and the fabrication process is relatively complex.
Another problem that arises with intravascular MRI antennas and intravascular guidewires used in conjunction with an MRI system is that the electrical conductors tend to pick up the RF signals from the MRI system. This results in a higher voltage on the conductors and unwanted heating of the conductors. One prior art method of dealing with such undesirable heating of conductors with respect to an intravascular MRI antenna employs two coaxial chokes in series on a triaxial cable. Each choke is prepared by soldering a short between the primary and secondary shields of the triaxial cable at one end and removing the secondary shield at the other end. A dielectric layer between the primary and secondary shields acts as a waveguide that translates the short into a high impedance at the open end of the choke. This reduces the heating of the conductors. However, since the shields are made from metallic conductors, some heating of the conductors still occurs.
In addition, general construction difficulties also present problems. Simply connecting the antenna back to the transmission line conductors in such a small environment is quite difficult.
The present invention addresses at least one of these and other problems and offers advantages over the prior art.